Cellular interactions which occur during an immune response are regulated by members of several families of cell surface receptors, including the tumor necrosis factor receptor (TNFR) family. The TNFR family consists of a number of integral membrane glycoprotein receptors many of which, in conjunction with their respective ligands, regulate interactions between different hematopoietic cell lineages (Smith et al., The TNF Receptor Superfamily of Cellular and Viral Proteins: Activation, Costimulation and Death, 76:959-62, 1994; Cosman, Stem Cells 12:440-55, 1994). Three receptor members of this family are (1) BCMA, B Cell Maturation Antigen (Gras et al., Int. Immunol. 17:1093-106, 1995 and Hatzoglou et al., J. Immunol., 165: 1322-30, 2000); (2) TACI, transmembrane activator and CAML-interactor (von Bülow and Bram, Science 228:138-41, 1997 and WIPO Publication WO 98/39361)) and (3) BAFF-R, also known as BLyS/BLyS receptor 3 (BR3), (Thompson et al., Science, 293: 2108-11, 2001). These receptors are known to bind one or both TNF ligands—B Lymphocyte stimulator (BLyS also known as BLyS, TALL-1, ztnf4 and THANK) (see, e.g., Shu et al., J. Leukoc. Biol. 65: 680-683 (1999)) and a proliferation-inducing ligand (APRIL) (see, e.g., Hahne et al., J. Exp. Med. 188: 1185-1190 (1998)). Specifically, TACI and BCMA are known to bind both BLyS and APRIL and BAFF-R binds only BLyS.
A number of BLyS and/or APRIL antagonists have been developed in order to block the binding of the ligands to the receptor members of the family, in order to block results of this binding which include but should not be limited to B cell co-stimulation, plasmablast and plasma cell survival, Ig class switching, enhanced B-cell antigen presenting cell function, survival of malignant B cells, development of B-1 cell function, B cell development beyond the T-1 stage, and complete germinal centre formation. Some of these molecules can also bind to and block the effect of APRIL on B cells and other components of the immune system (Dillon et al. (2006) Nat. Rev. Drug Dis. 5, 235-246). Molecules that have been developed to affect B cell function by interfering with BLyS and/or APRIL binding include BLyS antibodies such as Lymphostat-B (Belimumab) (Baker et al, (2003) Arthritis Rheum, 48, 3253-3265 and WO 02/02641); receptor-extracellular domain/Fc domain fusions proteins such as TACI-Ig, including one particular embodiment, atacicept (U.S. Patent Application No. 20060034852), BAFF-R-Fc (WO 05/0000351), and BCMA-Ig or other fusion proteins utilizing receptor extracellular domains. A further class of BLyS and/or APRIL antagonists include other molecules relying on BLyS binding ability to block binding to its receptors such as AMG 623, receptor antibodies, and other molecules disclosed in WO 03/035846 and WO 02/16312.
One understudied aspect of this ligand/receptor family is the fact that these ligands appear to exist in vivo not only as homotrimers (which would be expected for TNF family ligands), but also as BLyS/APRIL heterotrimers (HT) of uncharacterized stoichiometry. Using an extremely small and heterogenous sample set (i.e., 15 patients with 6 different autoimmune disease diagnosies between them), the existence of elevated HT in sera as compared to healthy controls was reported by Roschke et al. (J. Immunol. 169:4314-4321, 2002). However, there has been no association of the presence of elevated HT with any particular autoimmune disease, an analysis necessary to apply such findings to specific disease treatment methods nor has this finding been presented beyond anecdotally, i.e., with statistical significance.
Thus, there remains a need in the art for investigation into the biological activity of HT and the development of an assay to compare the levels of native HT to those of homotrimeric BLyS and APRIL in autoimmune patients. Furthermore, this assay allows identification of expression patterns of HT so that statistical associations with autoimmune disease can be identified, such as systemic lupus erythematosus (SLE). Such information is important for identifying individuals who have a propensity toward developing such autoimmune diseases, are in an active disease state, and for identifying those that will respond favorably to BLyS and/or APRIL antagonist treatment of these diseases. The present invention addresses this need by providing HT levels associated with autoimmune diseases and providing diagnostic tests determining the presence of this expression pattern, namely increased HT levels in serum for those suffering from autoimmune disease such as SLE as compared to levels seen in healthy controls.